1. Field of the Invention
This invention relates to lateral semiconductor devices, and more particularly to high voltage lateral devices with a reduced area configuration.
2. Description of the Related Art
High voltage lateral devices using very thin epitaxial or implanted layers are widely known. In these lateral devices, changes in the electric field distribution take place near the surface. RESURF (REduced SURface Field) technology has been developed to fabricate these devices. The basic structure generally consists of a high-ohmic P-type semiconductor substrate with a lightly-doped formed using this technique.
The critical characteristics for a high voltage lateral device include the breakdown voltage, the on-state resistance, and the overall area of the device. Optimization of those characteristics requires attention to design parameters, particularly doping concentrations, drift length or lateral separation between drain/high-voltage regions and source/low-voltage regions, and the shaping or radius of curvature of the N.sup.+ and P.sup.+ regions.
High voltage lateral devices are normally required to handle more than 300 volts. Conventional 2-dimensional analysis considers the necessary substrate doping and drift length to achieve the breakdown voltage for a particular device. Frequently, these lateral devices are formed with elongated N.sup.+ drain regions or source regions which terminate with a predetermined planform radius of curvature at the surface. However, the 3-dimensional effect of this curvature of the N.sup.+ drain region on the electric field is often not considered in 2-dimensional analysis, yet this curvature may be a limiting factor for the breakdown voltage of the device, according to H. Yilmaz in "Modeling and Optimization of Lateral High Voltage IC Devices to Minimize 3-D Effects," Electrochemical Society Proceedings on High Voltage and Smart Power Devices, Vol. 87-13. The curvature of the N.sup.+ drain/collector electrode enhances the electric field at the N-N.sup.+ junction. Therefore, actual breakdown voltage of lateral devices optimized by 2-dimensional simulation will be reduced by the N-N.sup.+ junction curvature. According to E. J. Wildi et al. in "Modeling and Process implementation of implanted RESURF Devices," Technical Digest 1982 IEDM, pp. 268-271, breakdown occurs first in a test device at the tip of an N.sup.+ region having a smaller radius of curvature than elsewhere in the device. The 3-D charge control model of Yilmaz suggests that curvature effects may be reduced with a consequent increase in breakdown voltage by either (1) increasing the N.sup.+ drain radius of curvature, or (2) increasing the drift length between the N.sup.+ drain and the source at the ends of the N-N.sup.+ junction where the curvature exists. The first method, increasing the N.sup.+ radius, has the effect of increasing the width of the body of the entire N.sup.+ drain region, as well as the overall area of the device if the optimum drift lengths are maintained around the widened N.sup.+ drain. The second method, i.e. maintaining the optimized drift length throughout the device while increasing drift length at the ends, is disadvantageous because the lengthened drift regions may significantly increase the on-state resistance of the device.
Therefore, there is a need for a high voltage lateral device that utilizes less area, yet maintains the breakdown voltage and on-state resistance of the device.